Assembly of two high-performance thermoplastic matrix composites by welding generally involves the use of known assembly techniques. Moreover, in order to heat the assembly zone to the desired temperature, it is possible to employ several known principles of heating such as heating by hot gas (stove, autoclave), by vibration (ultrasound, rotation), by induction, by resistance, etc. This type of assembly is notably described in the book by Michael J. Troughton with the title “Handbook of plastics joining” (Plastics Design Library). ISBN: 978-1-884207-17-4.
Conversely, to date, there is no known means for assembly of a thermoplastic matrix composite and a material of the elastomer type by welding. This absence seems to have its origin in the fact that since the elastomers do not have the property of passing reversibly from the solid state to the molten state, an assembly technique requiring heating of the materials to be assembled, at least at the level of the assembly zone, does not seem obvious to a person skilled in the art.
For certain applications, the known assembly techniques, allowing assembly of an element made of thermoplastic material with an element made of polymer material, do not give the required qualities, notably in terms of mechanical durability.
This is the case for example, as illustrated schematically in FIG. 1, for assembly of a shroud 11 (of metallic material or composite with thermoplastic or thermosetting matrix) on a tank 12 of thermoplastic matrix composite to constitute a propulsion stage of a powder launch vehicle, or a consumer gas tank.
In fact this assembly requires the insertion of a layer 13 of elastomeric material within the joint, said layer of elastomeric material having the role of absorbing the large differential strains that develop between the two structures. Therefore insertion of this interposed layer 13 of elastomeric material requires assembling this layer on the two elements of composite material that constitute the tank 12 and the shroud 11, and said assembly must necessarily be of high mechanical quality, owing to the stresses imposed on the assembly.
Now, in terms of mechanical durability, a welded joint is generally superior to a glued joint. In fact, after welding, there is no longer a differentiated interface between the two assembled parts, owing to diffusion of molecules from one part to the other, whereas in the case of a glued joint, there are as many interfaces as there are layers required for creating the joint, said layers mainly including, besides the layer of adhesive proper, the adhesion primers, as well as the treatments applied on the surfaces to be assembled.
Moreover, in the case of welding, we also no longer have the constraints due to preparation of the surfaces before assembly and protection of them against contaminants, these operations being necessary, or even imperative, in the case of assembly by gluing.